Warn on unused events/transitions

[Rich-Harris]

Follow-up to #1050. It doesn't do helpers, because that would require a bit more work that I can't be bothered to do right now.

Fixes #1051

[codecov-io]

Codecov Report

Merging #1052 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
+ Coverage   92.02%   92.05%   +0.02%     
==========================================
  Files         121      121              
  Lines        4376     4392      +16     
  Branches     1410     1415       +5     
==========================================
+ Hits         4027     4043      +16     
  Misses        148      148              
  Partials      201      201

Impacted Files	Coverage Δ
src/generators/server-side-rendering/index.ts	95.77% <ø> (ø)	⬆️
src/validate/html/validateElement.ts	89.36% <100%> (+0.47%)	⬆️
src/validate/index.ts	89.09% <100%> (+2.42%)	⬆️
src/generators/dom/index.ts	95.48% <100%> (+0.05%)	⬆️

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